The present invention generally relates to error handling in the field of communication systems and, more particularly, to error handling using automatic retransmission requests (ARQ) in digital communication systems that support multiple FEC coding and/or modulation schemes.
The growth of commercial communication systems and, in particular, the explosive growth of cellular radiotelephone systems, have compelled system designers to search for ways to increase system capacity without reducing communication quality beyond consumer tolerance thresholds. One technique to achieve these objectives involved changing from systems, wherein analog modulation was used to impress data onto a carrier wave, to systems wherein digital modulation was used to impress the data on carrier waves.
In wireless digital communication systems, standardized air interfaces specify most of the system parameters, including modulation type, burst format, communication protocol, etc. For example, the European Telecommunication Standard Institute (ETSI) has specified a Global System for Mobile Communications (GSM) standard that uses time division multiple access (TDMA) to communicate control, voice and data information over radio frequency (RF) physical channels or links using a Gaussian Minimum Shift Keying (GMSK) modulation scheme at a symbol rate of 271 ksps. In the U.S., the Telecommunication Industry Association (TIA) has published a number of Interim Standards, such as IS-54 and IS-136, that define various versions of digital advanced mobile phone service (D-AMPS), a TDMA system that uses a differential quadrature phase shift keying (DQPSK) modulation scheme for communicating data over RF links.
TDMA systems subdivide the available frequency into one or more RF channels. The RF channels are further divided into a number of physical channels corresponding to timeslots in TDMA frames. Logical channels are formed of one or several physical channels where modulation and coding is specified. In these systems, the mobile stations communicate with a plurality of scattered base stations by transmitting and receiving bursts of digital information over uplink and downlink RF channels.
The growing number of mobile stations in use today has generated the need for more voice and data channels within cellular telecommunication systems. As a result, base stations have become more closely spaced, with an increase in interference between mobile stations operating on the same frequency in neighboring or closely spaced cells. Although digital techniques provide a greater number of useful channels from a given frequency spectrum, there still remains a need to reduce interference, or more specifically to increase the ratio of the carrier signal strength to interference, (i.e., carrier-to-interference (C/I) ratio). Note that although the present invention is described in the context of measuring channel robustness in terms of C/I, those skilled in the art will appreciate that the carrier-to-noise ratio is also a commonly used measurement for channel robustness. For brevity, "C/I" is used throughout this text, but should be taken to mean "C/I and/or C/N".
In order to provide various communication services, a corresponding minimum user bit rate is required. For example, for voice and/or data services, user bit rate corresponds to voice quality and/or data throughput, with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of techniques for speech coding, channel coding, modulation scheme, and for a TDMA system, the number of assignable time slots per call.
Conventionally, different digital communication systems use a variety of linear and non-linear modulation schemes to communicate voice or data information. These modulation schemes include, for example, Gaussian Minimum Shift Keying (GMSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc. Typically, each communication system operates using a single modulation scheme for transmission of information under all conditions. For example, ETSI originally specified the GSM standard to communicate control, voice and data information over links using a GMSK modulation scheme to provide transmission and retransmission of information.
Depending on the modulation scheme used by a particular system, the throughput of a packet transmission scheme deteriorates differently as C/I levels decrease. For example, modulation schemes may use a different number of values or levels to represent information symbols. The signal set, i.e., amplitude coefficients, associated with QPSK, an exemplary lower level modulation (LLM) scheme, are illustrated in FIG. 1(a). By way of comparison, 16QAM is a higher level modulation (HLM) scheme having the signal set depicted in FIG. 1(b).
As can be seen in FIGS. 1(a) and 1(b), the minimum Euclidean distance between the coefficients in the LLM scheme is greater than the minimum Euclidean distance between coefficients in the HLM scheme for the same average signal power, which makes it easier for receive signal processing to distinguish between modulation changes in the LLM scheme. Thus, LLM schemes are more robust with respect to noise and interference, i.e., require a lower carrier-to-interference (C/I) level to achieve acceptable received signal quality. HLM schemes, on the other hand, provide greater user bit rates, e.g., 16QAM provides twice the user bit rate of QPSK, but require higher C/I levels.
More recently, however, dynamic adaptation of the modulation used for transmission in radiocommunication systems types has been considered as an alternative that takes advantage of the strengths of individual modulation schemes to provide greater user bit rates and/or increased resistance to noise and interference. An example of a communication system employing multiple modulation schemes is found in U.S. Pat. No. 5,577,087. Therein, a technique for switching between 16QAM and QPSK is described. The decision to switch between modulation types is made based on quality measurements, however this system employs a constant user bit rate which means that a change in modulation scheme also requires a change in channel bit rate, e.g., the number of timeslots used to support a transmission channel.
In addition to modulation schemes, digital communication systems also employ various techniques to handle erroneously received information, which techniques are not described in U.S. Pat. No. 5,577,087. Generally speaking, these techniques include those which aid a receiver to correct the erroneously received information, e.g., forward error correction (FEC) techniques, and those which enable the erroneously received information to be retransmitted to the receiver, e.g., automatic retransmission request (ARQ) techniques. FEC techniques include, for example, convolutional or block coding of the data prior to modulation. FEC coding involves representing a certain number of data bits using a certain number of code bits. Thus, it is common to refer to convolutional codes by their code rates, e.g., 1/2 and 1/3, wherein the lower code rates provide greater error protection but lower user bit rates for a given channel bit rate.
ARQ techniques involve analyzing received blocks of data for errors and requesting retransmission of blocks which contain any error. Consider, for example, the block mapping example illustrated in FIG. 2 for a radiocommunication system operating in accordance with the Generalized Packet Radio Service (GPRS) optimization which has been proposed as a packet data service for GSM. Therein, a logical link control (LLC) frame containing a frame header (FH), a payload of information and a frame check sequence (FCS) is mapped into a plurality of radio link control (RLC) blocks, each of which include a block header (BH), information field, and block check sequence (BCS), which can be used by a receiver to check for errors in the information field. The RLC blocks are further mapped into physical layer bursts, i.e., the radio signals which have been GMSK modulated onto the carrier wave for transmission. In this example, the information contained in each RLC block can be interleaved over four bursts (timeslots) for transmission.
When processed by a receiver, e.g., a receiver in a mobile radio telephone, each RLC block can, after demodulation and FEC decoding, be evaluated for errors using the block check sequence and well known cyclic redundancy check techniques. If there are errors after FEC decoding, then a request is sent back to the transmitting entity, e.g., a base station in a radiocommunication system, denoting the block to be resent.
The GPRS optimization provides four FEC coding schemes (three convolutional codes of different rate and one uncoded mode), but uses only one modulation scheme (GMSK). After one of the four coding schemes is selected for a current LLC frame, segmentation of this frame to RLC blocks is performed. If an RLC block is found to be erroneous at the receiver and needs to be retransmitted, the originally selected coding scheme must be used for retransmission.
Another example of ARQ techniques is found in published International Application PCT/FI96/00259. Therein, a digital telecommunications system is described wherein quality measurements associated with a connection are made based on the number of retransmissions requested. If the quality drops below a threshold, then a more efficient coding scheme is used to transmit information for that connection.
Although the aforedescribed adaptive systems attempt to adjust to quality changes associated with a radio channel, they each suffer from certain drawbacks and limitations. For example, the system described in U.S. Pat. No. 5,577,087 is limited to changes in modulation, does not address the added complexity associated with ARQ techniques and does not provide any flexibility in terms of block segmentation or user bit rate adjustment. Although the GPRS optimization and the aforedescribed PCT application address ARQ, the systems described therein are limited to changes in FEC coding. Moreover, the GPRS system does not permit changes in FEC coding for the retransmitted block and the FEC coding changes proposed in the PCT application affect the entire connection rather than simply the retransmitted block, which may be unnecessary in most cases.